1. Field of the Invention
The subject invention relates to a servo spot welding control system including a robot and multiple electrodes for efficiently welding a pair of workpieces together. The subject invention also relates to a method for welding the pair of workpieces together.
2. Description of Related Art
Various welding control systems are known in the art. One example of a welding control system is disclosed in U.S. Pat. No. 5,111,019. More specifically, the ""019 patent discloses a welding control system for resistance spot welding. A pair of transformers and a pair of spot welding guns are attached to a wrist of a robot. Each welding gun has a pair of electrodes, a fixed electrode and a movable electrode. A fluid-activated cylinder, such as an air cylinder, is used to actuate movement of the movable electrode during welding.
The ""019 patent does not disclose the use of servomotors to actuate the movable electrodes. Servomotors are more desirable than cylinders for several reasons. First, the pressure on the workpieces can be controlled more accurately, allowing higher quality welds and less damage to the workpieces. Furthermore, servomotors typically operate more quickly than cylinders, which reduces overall cycle time. Yet another reason is that servomotors are quieter than air cylinders since there is no pneumatic exhaust associated with the servomotors. The ""019 patent also does not disclose the use of a single transformer and a controller to sequence electric current between the electrodes. A single transformer reduces weight on the robot arm and cost of the overall system.
Another example of a welding control system is disclosed in U.S. Pat. No. 5,313,040. The ""040 patent also discloses a welding control system for resistance spot welding. Multiple weld guns are mounted in fixed locations. Only one weld is created at one time. The ""040 patent does not disclose a weld gun with multiple welding electrodes that is mounted on a robot that can be controlled and manipulated in many different positions and orientations. Mounting the weld gun on the robot provides greater flexibility than multiple weld guns that are mounted in fixed locations.
A final example of a welding control system is disclosed in U.S. Pat. No. 5,340,960. The ""960 patent also discloses a servo weld control system for resistance spot welding. The system includes a robot, a welding gun, a robot controller, and a servomotor. The welding gun has a single pair of electrodes, a fixed electrode and a movable electrode. The servomotor actuates the single movable electrode. The ""960 patent does not disclose multiple pairs of electrodes on one weld gun or a controller and method to sequence electric current to the multiple pairs of electrodes. Having multiple pairs of electrodes can improve cycle time since the robot need not always move to accomplish multiple spot welds. In addition, having multiple pairs of electrodes on one weld gun can reduce the dependence on multiple weld control systems at a single location.
Due to the deficiencies in the welding control systems identified above, there remains an opportunity to introduce a welding control system with faster cycle times, less equipment costs, greater flexibility, improved weld quality, and reduced damage to workpieces.
The invention provides a servo spot welding control system and method for efficiently welding a pair of workpieces together. The system includes a robot and a weld gun. The weld gun is movable about the workpieces, and the weld gun is mounted to the robot. The weld gun includes at least two pairs of electrodes, each pair including a stationary electrode and a movable electrode. A first servomotor is operatively connected to one of the movable electrodes and a second servomotor is operatively connected to the other of the movable electrodes. This arrangement allows for independent control of each movable electrode. The first servomotor clamps the workpieces between one of the pairs of electrodes and the second servomotor clamps the workpieces between the other of the pairs of electrodes. The system also includes a transformer. The transformer supplies electric current that flows through the pairs of electrodes for welding the workpieces together. The system further includes a controller to sequence the electric current between the pairs of electrodes.
The method of the subject invention utilizes the servo spot welding control system to weld the workpieces together. The method includes the step of moving the robot to dispose the weld gun about the workpieces. Electrical current is supplied to the first and second movable electrodes from the transformer. The workpieces are clamped between the first pair of electrodes on the weld gun. This allows the electric current to flow through the first movable electrode, the workpiece, and the first stationary electrode to create a first spot weld. The workpieces are released from the clamp of the first pair of electrodes. Upon release, the electric current is prevented from flowing. The workpieces are clamped between the second pair of electrodes on the weld gun. The clamping of the workpieces between the second pair of electrodes is done without additional movement of the robot. The clamping then allows the electric current to flow through the second movable electrode, the workpiece, and the second stationary electrode. As a result, a second spot weld is created, and the workpieces are then released from the clamp of the second pair of electrodes to prevent the electric current from flowing. It is to be appreciated that the robot could move the weld gun slightly between the first spot weld and the second spot weld. This may occur, for example, when a curved workpiece is to be welded.
The servo spot welding control system and method of the subject invention provide several advantages over the related art. This system and method provide faster cycle times than robot-mounted servo weld guns having only one pair of electrodes. Faster cycle times are accomplished because the robot does not have to reposition itself between each individual spot weld. The system and method also provide less equipment costs. Those skilled in the art will appreciate that it often takes two complete prior art spot welding systems, with two robots and one pair of electrodes per robot, to perform spot welds within an allotted cycle time. In the present invention, only one servo spot welding control system is necessary to accomplish the same number of spot welds as two systems of the prior art. The system and method also provide improved weld quality and less damage to the workpieces. The servomotor driven electrodes provide very accurate and controlled pressure to the workpieces. This results in better quality spot welds as compared to spot welds that are provided by electrodes that are driven by fluid-actuated cylinders. Also, servomotor driven electrodes can provide position and force feedback, synchronization with robot movement, and a complete range of electrode movement. Finally, having electric current supplied to multiple electrodes by a single transformer reduces the payload of the robot and allows the robot to move with faster accelerations between spot welds. These faster accelerations allow short cycle times for better productivity.